Category Modern Science

Define Light and explain its main features?

LIGHT

Light is a kind of energy. It is the form of energy that our eyes can detect, enabling us to see. It is produced by very hot things – the Sun, fire and the tiny wire inside electric light-bulbs. Certain animals also have light-producing organs.

Light from the Sun is essential to life on Earth. Some creatures live off minerals in the ocean depths but these are exceptions. Most plants use sunlight to make their food. All plant-eating animals, together with other animals that eat plant-eaters, also therefore depend on sunlight.

Light rays can only travel in straight lines. If they strike an object which does not allow light to pass through it (an opaque object), a shadow is cast on the unlit side. Light can be reflected, however. Light reflected from objects allows us to see them. Light rays strike and bounce off a flat, shiny surface like a mirror at the same angle. This enables us to see our reflection.

THE SPEED OF LIGHT

When we switch on an electric light, it seems that the room is filled with light instantaneously. But light rays do take time to travel from their source. They travel extremely quickly: about 300,000 kilometres (or seven-and-a-half times around the world) per second in outer space. The speed of light is, in fact, the speed limit for the Universe: nothing can travel faster. Light waves are able to travel through empty space – a vacuum – whereas sound waves cannot. Light actually moves less quickly through air, water or glass than through empty space.

Because stars are very far from Earth – at least thousands of billions of kilometres – astronomers measure their distances in light years, the amount of time it takes for light to travel to us from them.

REFRACTION OF LIGHT

Light rays bend, or refract, when they pass through different transparent materials. This is because light travels at different speeds through different materials. At the boundary between two materials, for example, air and water, the light changes speed slightly and is refracted from its straight path. You can see this effect when looking at the bottom of swimming pool. It looks much shallower than it really is.

FOCUSING LIGHT

A lens, a shaped piece of glass or plastic, can bend light, either spreading it out or bringing it closer together. A convex lens, one that is thicker in the middle than at the edge, brings light rays together at a single point called a focus. The eye contains a natural convex lens which focuses an image on to the retina at the back of the eye. If you hold a convex lens so that the object you are looking at lies between the lens and the focus, the object will appear larger and further from the lens than it really is. A simple magnifying glass is a convex lens, and is useful for studying minute detail as, for example, on a postage stamp or a tiny insect or flower.

A concave lens is the opposite of a convex lens: it is thicker around the edge than in the middle. This kind of lens diverges (spreads out) light rays. It is used in glasses to correct short-sightedness.

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What should I know about Electricity?

 

ELECTRICITY

One of the most useful forms of energy in today’s world is electricity. It is transportable, which means it can be carried long distances by wires and cables. It is convertible, being changed into many other forms of energy, such as light from an electric light-bulb, and movement in an electric motor. It is also controllable. We can turn it on and off with a switch, or up and down with a knob. When a city suffers a power cut and falls still and silent, we realize how much we depend on electricity.

Electricity is the movement of electrons, the negative particles around the nucleus of an atom. Most metals, especially silver and copper, have electrons that can move easily from atom to atom, so they are good carriers or conductors of electricity. Electrons are pushed along the conductor by a battery or generator. But they flow only if they have a complete pathway of conductors called a circuit. Flowing electricity is known as electric current.

In substances such as rocks, wood, plastics, rubber and glass the electrons do not move easily. These materials prevent the flow of electricity and are known as insulators, but they may gain or lose electrons on their surface as a static electric charge.

            Static electricity is produced when electrons are separated from their atoms. On a comb it attracts bits of paper. In the sky it causes lightning!

            Electric current flows along a wire as electrons which detach from the outermost parts of their own atoms and jump or hop along to the next available atoms.

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Could you please tell something interesting about Colours and World of Colours?

COLOUR

One of the main features of light is colour. If light were just pure white, our whole world would be black and white and shades of grey. But white light is not pure. It is a mixture of all the colours of the rainbow which are known as the spectrum of light.

Colours exist because light is in the form of waves and not all the waves have the same wavelength. Some are slightly longer than others, and these we see as red. Light waves of medium wavelength appear to our eyes as green. We see the shortest light waves as violet. A leaf is green because its surface absorbs all the colours in white light except green, which it reflects into our eyes. A red flag absorbs all colours except red. Objects that reflect all colours are white.

The colour wheel shows how the different colours of light add up to make white light. When you spin the wheel the colours whirl around so fast that the eye cannot follow them. Inside the eye each colour merges with the others so the eye sees all the colours at once – and all colours of light added together make white light.

The different colours of light are seen when white light is split up using a prism, an angled block of transparent material such as clear glass or plastic. As the light waves pass into and then out of the prism they are bent or refracted. Longer waves of red light refract least. Shorter waves of violet light refract most. The other colours spread out between. A raindrop works as a natural prism. Millions of raindrops split sunlight and form a rainbow in the sky.

ADDING COLOURS

We see colours in books and on screens such as the television, in different ways. A television or computer screen has thousands of tiny dots that glow and give out light. These dots have actually only three colours – red, green and blue. These colours are known as the primary colours of light. Added to each other in different combinations and brightness they can make any other colour. For example, red and green together make the colour yellow. Red and blue produce the pinky colour known as magenta. Blue and green form cyan, a type of turquoise. The three primary colours of red, blue and green added together make white light.

On the screen of a computer or TV the dots are arranged in groups known as pixels. The different colours of dots flash on and off in different combinations and shine with different brightnesses. From a distance, the eye cannot see the individual dots. They merge to produce larger areas of colour. When all the red dots on an area of the screen shine, that area looks red. When all three colours of dots in an area of the screen shine brightly, that area looks white. Also the dots flash on and off many times each second, again too fast for the eye to follow. So they merge together in time to produce multi-coloured, moving pictures.

SUBTRACTING COLOURS

Coloured pictures in a book are made like those on a screen, using tiny coloured dots that merge together. The dots are inks made with coloured substances called pigments. There are three primary pigment colours – yellow, magenta and cyan. They work in the opposite way to light colours. They do not add together, but take away or subtract. A yellow dot takes away all colours of light except yellow which it reflects. The other two dots do the same for their colours. By taking away individual colours, the dots merge to produce areas of other colours. All three dots together make black.

            The wolf’s mask is realistic and frightening. Yet it is printed using tiny dots of only three colours. They can be separated as magenta, cyan and yellow. To save on coloured inks some parts of the page, like these words, are printed with ready-made black ink.

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How does Transfer of Heat take Place?

HEAT MOVES

Heat can move around and between objects in three main ways. One is conduction, when heat energy passes between two objects in physical contact. When you touch an object to see how warm it is, you receive some of its heat by conduction. A second way is by convection. This only happens in liquids and gases. As some of the atoms or molecules receive heat energy and become warm they spread out more. The heated part of the liquid or solid is now less dense than its cooler surroundings so it rises or floats. As it rises, it carries its heat energy in the form of convection current. You can feel this as warm air rising from a central heating radiator.

The third way that heat moves is by radiation. It is in the form of infrared waves which are part of a whole range of waves, including radio waves, light and X-rays, known as the electromagnetic spectrum. Conduction and convection both need matter to transfer heat. Radiation does not. Infrared waves can pass through space, which is how the Sun’s heat reaches Earth.

Like light waves, infrared waves reflect from light-coloured or shiny surfaces. On a hot day, light-coloured clothes reflect the Sun’s warmth and keep you cooler than dark clothing, which absorbs the warmth. Substances that slow down conduction and convection, such as wood, plastic and glass fibre, are called thermal insulators. Layers of fat, or blubber in a whale, are good insulators.

The faster an aircraft goes, the greater the heat from friction with air. Very fast planes like the X-15 rocket have special heat-radiating paint that gives out heat as fast as possible, to prevent the metal skin of the plane melting at high speed.

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How would you distinguish between Pitch and Volume of Sound?

PITCH AND VOLUME

Sound has two important features. One is pitch. A low-pitched sound is deep, like a roll of thunder or a booming big drum. A high-pitched sound is shrill, like a snake’s hiss or the tinkle of a triangle. Pitch depends on the frequency of sound waves – the number of waves per second. High-pitched sounds have high frequencies.

Some sounds are so high-pitched that our ears cannot detect them. They are known as ultrasounds. Many animals, like dogs and bats, can hear ultrasounds.

The second important feature of sound is its loudness or volume. Some sounds are so quiet that we can only just hear them, like a ticking watch or the rustling of leaves. Other sounds are so loud, like the roar of engines or the powerful music in a disco, that they may damage the ears. Sound volume, or intensity, is measured in units called decibels (dB). Sounds of more than 80-90 decibels can damage our hearing.

            An ultrasound scanner beams very high-pitched sound waves into the body. The echoes are analyzed by a computer to form an image, the baby in the womb.

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How would you explain Heat?

HEAT

How warm is the weather today? It may be cold and wintry or hot and summery. Heat is a vital part of our lives. We need to keep our bodies comfortably warm with clothing, especially in cold conditions. If body temperature falls from its normal 37°C to below about 30°C, fatal hypothermia may set in.

We cook our food with heat using gas or electricity. Countless machines and industrial processes use heat, from making pottery or a photocopy to a steelworks or power station. Heat is also given off as a waste form of energy by many processes. In a power station most of the heat is used to generate electricity, but some is released as clouds of steam from huge cooling towers.

Heat is a type of energy – the vibrations of atoms and molecules. The more an atom moves or vibrates, the more heat or thermal energy it has. In a solid, the atoms have fixed central positions but each atom vibrates slightly about its central position, like a ball tied to a nail by elastic. Heat the solid and the atoms vibrate more. When they have enough vibrations, the atoms break from their fixed positions (the “elastic” snaps), and they move about at random. The solid has melted into a liquid. Heat it more and the atoms fly further apart. The liquid becomes a gas.

 TEMPERATURE

Cold is not the presence of something that opposes heat, but simply the lack of heat. Temperature is not the same as heat. Heat is a form of energy, while temperature is a measure of how much heat energy a substance or object contains. A slice of apple pie at 40°C contains more heat energy than a same-sized slice of the same pie at 30°C. We can judge its temperature quite accurately when we touch the slice with our skin, and especially with our fingertips or lips. But this judgement is only safe within a certain range. Temperatures greater than about 50°C or lower than about -10°C cause pain and may damage the skin. We measure temperatures accurately using devices called thermometers.

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